Gatekeeper was introduced in Mountain Lion and OS X Lion v10.7.5. It’s used to help protect your Mac from malware and misbehaving apps downloaded from the Internet.
The safest and most reliable place to download and install apps is via the Mac App Store.
For apps that are downloaded from places other than the Mac App Store, developers can get a unique Developer ID from Apple and use it to digitally sign their apps. The Developer ID allows Gatekeeper to block apps created by malware developers and verify that apps haven’t been tampered with since they were signed. If an app was developed by an unknown developer—one with no Developer ID—or tampered with, Gatekeeper can block the app.
Apple have announced some changes for the forthcoming 10.9.5 update for Mavericks. For an application to remain trusted, developers will have to make sure the code is signed using OS X Mavericks 10.9 or later and create a v2 signature. This new requirement will also be a requirement for OS X 10.10 Yosemite. Applications signed using these new methods will also still work for older versions of OS X, such as 10.7.5 to 10.8.
This could mean some apps, that Gatekeeper previously allowed to run, will now generate a warning until their digital signatures have been updated. As a temporary work around, users can right-click the app and choose “Open” from the contextual menu to bypass Gatekeeper for that application. Obviously only perform the action if you are 100% sure of the authenticity of the app.